Pushing the Limits on Metal-Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for o-Xylene Isomerization and Disproportionation

Article abstract of DOI:10.1021/jacs.8b04059

Acid-catalyzed skeletal C-C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Br?nsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal-organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), ³¹P NMR, N₂ physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WO_x clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WO_x-ZrO₂, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal-organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization.

Full text of DOI:10.1021/jacs.8b04059

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Article
Pushing the Limits on Metal–Organic Frameworks as a
Catalyst Support: NU-1000 Supported Tungsten Catalysts
for o-xylene Isomerization and Disproportionation
Sol Ahn, Scott L Nauert, Cassandra T. Buru, Martino Rimoldi, Hyeju Choi,
Neil M. Schweitzer, Joseph T. Hupp, Omar K. Farha, and Justin M Notestein
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04059 • Publication Date (Web): 17 Jun 2018
Downloaded from http://pubs.acs.org on June 18, 2018
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Pushing the Limits on Metal–Organic Frameworks as a Catalyst Support:
NUꢀ1000 Supported Tungsten Catalysts for oꢀxylene Isomerization and
Disproportionation
†
†
‡
‡
‡
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Sol Ahn, Scott L. Nauert, Cassandra T. Buru, Martino Rimoldi, , Hyeju Choi, Neil M.
†
‡
*†‡§
*†
Schweitzer, Joseph T. Hupp, Omar K. Farha,
and Justin M. Notestein
†
‡
Department of Chemical and Biological Engineering, and Department of Chemistry, Northwestern University,
2145 Sheridan Road, Evanston, Illinois 60208, United States
§
Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
■
ABSTRACT
Acidꢀcatalyzed skeletal CꢀC bond isomerizations are important benchmark reactions for the
petrochemical industries. Among those, oꢀxylene isomerization/disproportionation is a probe
reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density
and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated
within NUꢀ1000, a Zrꢀbased metal–organic framework (MOF), as a catalyst for oꢀxylene
31
isomerization at 523 K. Extended Xꢀray absorption fine structure (EXAFS), P NMR, N2
physisorption, and XRD show that the catalyst is structurally stable with timeꢀonꢀstream, and
x
that WO clusters are necessary for detectable rates, consistent with conventional catalysts for
the reaction. PTA and framework stability under these aggressive conditions requires
maximal loading of PTA within the NUꢀ1000 framework; materials with lower PTA loading
lost structural integrity under the reaction conditions. Initial reaction rates over the NUꢀ1000ꢀ
supported catalyst were comparable to a control WO ꢀZrO , but the NUꢀ1000 composite
x
2
material was unusually active toward the transmethylation pathway that requires two adjacent
active sites in a confined pore, as created when PTA is confined in NUꢀ1000. This work
shows the promise of metal–organic framework topologies in giving access to unique
reactivity, even for aggressive reactions such as hydrocarbon isomerization.
KEYWORDS: metal–organic framework, solid acid, polyoxometalate, phosphotungstic acid,
heterogeneous catalysis, supported catalyst, o-xylene, isomerization, disproportionation
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■ INTRODUCTION
Isomerization and disproportionation reactions of hydrocarbons such as hexane and xylene
are crucial reactions in the production of fuels and basic petrochemicals, and these reactions
have also long been model reactions that provide structural information on solid acid
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catalysts. Tungstated zirconia (WO ꢀZrO ) is one example of a material capable of carrying
4
ꢀ5
6
out a wide range of solid acid catalysis such as alkane isomerization, alcohol dehydration
7
8ꢀ10
and etherification, and oꢀxylene isomerization.
WO ꢀZrO is typically synthesized by
x
2
impregnation on amorphous supports and subsequent calcination. These materials have very
11
strong structure sensitivity, in which the structure of the active material strongly influences
the rate. As such, the use of conventional syntheses that cause heterogeneity of active sites
can complicate the understanding of the material’s overall reactivity. Phosphotungstic acid
3
(PTA, H PW12O40) is a Keggin polyoxometalate (POM) cluster that can provide the tungsten
oxide active sites for the aforementioned model reactions owing to its activity for acid
catalysis. For instance, pure PTA has been reported for depolymerization of
12
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polytetrahydrofuran and alkylation of toluene. Researchers have extensively studied PTA
as heterogeneous acid catalysts on various supports. Kukovecz et al. reported solꢀgel
1
4
incorporation of PTA on silica for alkene isomerization, Devassy et al. impregnated PTA on
15
zirconia for phenol alkylation, and Macht et al. deposited PTA on SiO via incipient wetness
2
1
6
impregnation for alcohol dehydration.
Separately, many researchers have been studying metal–organic frameworks (MOFs) as
catalyst supports potentially capable of creating uniform active sites placed in controlled pore
17ꢀ20
environments.
It is generally hypothesized that catalysts with more uniform and isolated
2
1ꢀ22
active sites will lead to better understanding of the catalytic surface phenomena.
In
addition, MOFs have various applications in many fields such as gas storage and
23ꢀ27
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29ꢀ30
separation,
1000, possesses outstanding chemical and hydrothermal stability relative to many MOFs.
Further, the hexaꢀzirconium (Zr ) nodes are only 8ꢀconnected via tetracarboxylate (1,3,6,8ꢀ
drug delivery, or sensors.
In particular, the zirconiumꢀbased MOF, NUꢀ
3
1ꢀ32
6
4
ꢀ
tetrakis(pꢀbenzoate)pyrene, (TBAPy) ) linkers, providing four additional grafting sites on
each node in the form of four terminal and four bridging hydroxyl groups. The size of the
tetracarboxylate linkers places each Zr
6
cluster at least 1.0 nm from one another, ensuring site
3
3ꢀ34
isolation for small grafted species such as single metal ions.
Finally, three distinct pore
openings are created with csqꢀnet topology. Hexagonal and triangular pores of 3.1 and 1.2 nm,
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respectively, are aligned along the aꢀaxis, while 1.0 nm rhomboid pores are aligned along the
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cꢀaxis (Figure 1).
Recently, Zhang et al. encapsulated PTA in MILꢀ101, a Crꢀbased MOF, for sugar
36
dehydration. Also, Wang et al. synthesized WO
x
ꢀZrO
2
using UiOꢀ66 as a precursor for
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38ꢀ43
acetalization of benzaldehyde. These studies among others
have furthered the
development of MOFꢀbased Brønstedꢀacidic catalysts, but in some cases, MOFs are unstable
under the reaction conditions. In this study, we encapsulate a high weightꢀloading of PTA
within NUꢀ1000 and show that the POM and MOF structures remain intact following the
representative strong acidꢀcatalyzed reaction of oꢀxylene isomerization/transmethylation. The
structures and reactivities of NUꢀ1000ꢀsupported PTA catalysts are compared to a reference
x 2 x
WO ꢀZrO and to a WO ꢀloaded NUꢀ1000 synthesized by an alternate route. To the best of
our knowledge, this work provides the first example of an acidꢀcatalyzed, hydrocarbon
skeletal isomerization reaction – a class of important benchmark strong Brønsted acidꢀ
catalyzed reactions for the petrochemicals industries – occurring within a MOF.
6
Figure 1. Crystal structure of NUꢀ1000. The structures of Zr node (top left) and organic
linker (bottom left) are given. Dark green=Zr, red=O, gray=C, white=H
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■ RESULTS AND DISCUSSION
Synthesis and Characterization.
Table 1 summarizes the bulk properties of the Wꢀloaded catalysts synthesized in this study.
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The reference WO ꢀZrO possesses 16.5 wt% W, corresponding to 1.6 W/nm , or 22% of a
x
2
2
monolayer (0.22 W/Zrsurf), when considering the N physisorption surface area and Zr atom
surface density of the original support. WꢀSIM (Solution phase grafting in MOFs) is
synthesized by a selfꢀlimiting method that gives a similar W loading by mass, but a much
2
lower surface density based on total N physisorption surface area, due to the high total
surface area of the MOF. However, surface density may be more useful if normalized to
surface zirconium atoms, since W will not be grafted directly on the organic linkers.
Assuming that all the Zr atoms in the nodes are accessible, WꢀSIM reaches 0.41 W/Zrsurf or
2.5 W per Zr
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node. This loading is typical of deposition of mononuclear precursors by
grafting or atomic layer deposition (ALD) within NUꢀ1000, which typically does not exceed
21ꢀ22, 44ꢀ47
1
4
metals per Zr
complete removal of the precursor ligands (tertꢀbutylimido and dimethylamino) and thus the
conversion of WꢀN and W=N into a supported WO . Finally, two different loadings of
6
node when the MOF is intact.
In WꢀSIM, H NMR confirmed the
x
PW12@NUꢀ1000 are synthesized by exposure of NUꢀ1000 to aqueous PTA solutions. To
synthesize 0.5 PW @NUꢀ1000, NUꢀ1000 is exposed to 0.5 equivalents of PTA to one Zr
12
6
2
node resulting in loadings of 30.6 wt% W, 0.4 W/nm total N
2
physisorption surface area, and
0.9 W per surface Zr. To obtain higher POM loading, 0.9 PW12@NUꢀ1000 was synthesized
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with two equivalents of PTA per Zr , following a previously published procedure. Our team
6
previously demonstrated the uniform distribution of PTA within the NUꢀ1000 in the case of
4
8
0
.9 PW12@NUꢀ1000 using SEMꢀEDX; we confirm here a similarly uniform distribution of
PTA within 0.5 PW @NUꢀ1000 as well (Figure S1). With exposure to excess PTA, the
12
2
material contains 44.9 wt% W, corresponding to 0.5 W/nm , or 1.7 W/Zrsurf. This is
6 6
equivalent to 10.3 W per Zr node or 0.86 POM per Zr node; approximately 1 POM per node
appears to be the maximum value. After loading the Wꢀcontaining species, WꢀSIM and the
two PW12@NUꢀ1000 samples have lower surface areas than the original NUꢀ1000 based on
gravimetric surface area, as expected, and these composites still possess the characteristic
mesopores (step around P/P
o
=0.2ꢀ0.25 in Figure 2a and pore widths in Figure 2b) as well as
micropores of NUꢀ1000. Additionally, diffuse reflectance infrared Fourier transform (DRIFT)
ꢀ1
spectra (Figure S2) show a decrease in the OꢀH stretch (3670 cm ) from the nonꢀH bonded
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hydroxyl and aqua ligands for both WꢀSIM and 0.9 PW12@NUꢀ1000 relative to the starting
material, consistent with anchoring/encapsulation of WO clusters via the hydroxyl ligands of
x
NUꢀ1000.
a
Table 1. Summary of catalysts.
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BET
Volumetric
Surface Surface
W loading
6
wt% W] [W/Zr node]
[
Catalyst
Surface area Surface area density density
2
2
3
2
[
m /gcatalyst] [m /cm NUꢀ1000] [W/nm ] [W/Zrsurf]
ZrO
WO
NUꢀ1000
WꢀSIM
2
n/a
16.5
n/a
n/a
n/a
n/a
2.5
5.4
10.3
270b
n/a
n/a
1050
560
840
700
n/a
1.59
n/a
0.20
0.37
0.54
n/a b
0.22
n/a c
0.41c
0.90
x
ꢀZrO
2
125
2160
c
17.0
930
c
0
0
.5 PW12@NUꢀ1000 30.6
.9 PW @NUꢀ1000 44.9
Values were reproducible to within ± 3.5 % from three different catalyst synthesis batches
Calculated based on surface Zr density of zirconia,
Assuming every Zr of NUꢀ1000 node is exposed on the surface
1150
c
c
740
1.72
1
2
a
b
c
49ꢀ50
2
7.34 Zr/nm
Figure 2. (a) N
2
physisorption adsorption/desorption isotherms and (b) DFT (carbon slit pore
2
N 77 K kernel) pore size distributions of NUꢀ1000 (black), WꢀSIM (red), 0.5 PW12@NUꢀ
1000 (green), and 0.9 PW12@NUꢀ1000 (blue). Circles: as synthesized, squares: after catalysis.
Considering the W loadings and surface densities alone, the two PW12@NUꢀ1000 catalysts
and the control WO
x 2 x
ꢀZrO are expected to contain some active WO clusters, while the Wꢀ
SIM may possess highly dispersed W atoms that would not be catalytically active. In addition,
x 2
the relatively low diffuse reflectance UVꢀvis (DRUVꢀvis) edge energy of WO ꢀZrO of 2.95
8
eV (Figure S3) is consistent with the presence of WO
x
clusters. DRUVꢀvis is unfortunately
not diagnostic for NUꢀ1000 supported catalysts due to the stronglyꢀabsorbing organic linkers
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that overlap the WOx edge.
PXRD patterns (Figure 3a) show that the longꢀrange order of the MOF framework in Wꢀ
SIM and 0.9 PW12@NUꢀ1000 are maintained upon W deposition and after oꢀxylene
isomerization reaction conditions. Peak broadening observed in the case of WꢀSIM after
catalysis might indicate partial loss of structural integrity. For the asꢀsynthesized PW @NUꢀ
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1000 catalysts, these patterns have been previously assigned to the presence of POM clusters
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located in the small triangular channels of NUꢀ1000.
Figure 3. Bulk structure characterizations of catalysts (a) PXRD patterns, and (b) Solidꢀstate
3
1
P CP MAS NMR
x
NMR and EXAFS help indicate the possible structures of the WO species present in these
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materials. Solidꢀstate P cross polarization magic angle spinning (CP MAS) NMR spectra
(Figure 3b) confirm that the POM structure is intact upon synthesis and after the reaction.
Pure (solid) POM presents a sharp peak at ꢀ15 ppm that is attributed to the symmetric central
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P atom, and very small feature at ꢀ14 ppm. Kozhevnikov et. al. reported a feature at ꢀ14
5
2
ppm that increased with decreasing POM loading on silica. In our case, this feature is
intense in the asꢀsynthesized catalyst and diminishes significantly after the reaction. Given
that the material will dehydrate during reaction, we suggest that this feature is related to POM
in a hydrated or hydroxylꢀrich environment. We attribute the feature at ꢀ16 ppm in both fresh
and used catalysts to the direct interaction of POM and the Zr nodes of NUꢀ1000.
6
x
EXAFS results support the presence of WO clusters in PW12@NUꢀ1000 and isolated W
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atoms in WꢀSIM. Although the W coordination shells could not be readily fit in a quantitative
manner, a strong feature centered around 3.2 Å in Figure 4a has been assigned to WꢀOꢀW
5
3
scattering in WO
x
clusters. On the other hand, WꢀSIM does not present strong features in
cluster formation (Table 1). Furthermore, the two
this regime, indicating negligible WO
x
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peaks at 1.2 and 1.5 Å in Figure 4b are assigned to W=O and W–O, respectively. The
stronger relative intensity of W=O to W–O in WꢀSIM compared to 0.9 PW12@NUꢀ1000 is
x x
further consistent with isolated WO sites in WꢀSIM and extended WO domains in 0.9
PW @NUꢀ1000.
1
2
x
The low surface density and the EXAFS are both indicative of isolated WO groups in Wꢀ
SIM, which in turn predicts that WꢀSIM would have negligible reactivity toward oꢀxylene
8
isomerization. Isolated metal oxides cannot readily delocalize charge, resulting in weak or
absent Brønsted acid behavior.
Figure 4. Rꢀspace EXAFS spectra of (a) 0.9 PW @NUꢀ1000, and (b) WꢀSIM
1
2
Catalysis.
Previous reports state that oꢀxylene catalysis rates and selectivities are strongly dependent on
8
ꢀ9
the local WO
x x
structure. Rates per W atom increase up to monolayer capacity for WO ꢀ
ZrO and then fall at higher loadings as WO becomes sterically inaccessible. Similarly, it has
2
x
2
been reported that moderate POM loadings on TiO surfaces are more favorable than either
wellꢀdispersed POMs or large aggregates of POMs. In the former, strong interactions with the
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support limit the acidity, while in the latter, extended clusters of POMs limit acid site
5
4
accessibility.
a
Table 2. Summary of catalysis.
c
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Selectivity [%]
Initial TOF
Catalyst
WO ꢀZrO
NUꢀ1000
WꢀSIM
ꢀ1
ꢀ1 b
[
mol mol
W
hr ]
Isomerization Disproportionation
x
2
0.11
< 0.01d
< 0.01
100
ꢀ
0
ꢀ
d
ꢀ
ꢀ
e
d
0.5 PW12@NUꢀ1000
0.9 PW12@NUꢀ1000
< 0.01
ꢀ
ꢀ
0.16
17
83
a
5
0ꢀ100 mg catalyst (to match amount of W) to reach <10% conversion, 523 K in 10
sccm oꢀxylene/Ar and 10 sccm H at nominal 1 bar pressure. Turnover frequencies are
2
ꢀ1
reproducible to ± 0.01 hr and selectivities are reproducible to ± 2% across triplicate
runs of three different catalyst synthesis batches. See experimental methods for full
details.
b
c
Extrapolated to 0 min from the trend of oꢀxylene conversion between 80 and 240 min.
Isomerization (mꢀ and pꢀxylene) or disproportionation (toluene and trimethylbenzene)
products divided by total products at 240 min timeꢀonꢀstream (TOS).
d
e
ꢀ1
TOF of 0.01 hr reflects the detection limit of FID under the chosen conditions.
Catalysts 0.3, 0.4, and 0.7 PW12@NUꢀ1000 catalysts are similarly unreactive.
Table 2 compares reactivities and selectivities of the four catalysts at 523 K. This
temperature was chosen because product yields were below detectable limits for all NUꢀ1000
based catalysts at lower temperatures (423 and 473 K). Future work will address whether this
represents a critical temperature to activate the material or a high apparent activation energy.
At one extreme, WꢀSIM is inactive for oꢀxylene catalysis at 523 K, as predicted by its site
isolation. At the other extreme, WO
S4), consistent with its wellꢀestablished use in this reaction. Like WꢀSIM, 0.5 PW12@NUꢀ
000 also showed no activity at any time, but in this case, this result conflicts with the
x 2
ꢀZrO is active and stable with timeꢀonꢀstream (Figure
1
expected strong acidity of the POM unit, indicating that the POM and/or MOF structure
collapsed during the catalyst preꢀheating to 523 K or immediately upon the start of the
reaction. In contrast, 0.9 PW @NUꢀ1000 shows initial reactivity (per W atom basis) even
12
higher than WO
x
ꢀZrO
2
. To understand the difference between these two catalysts, we note
2
that with the absence of activity, 0.5 PW12@NUꢀ1000 lost most of its surface area (70 m /g)
and porosity (Figure 2). Likewise, the longꢀrange NUꢀ1000 crystal structure of 0.5
PW12@NUꢀ1000 collapsed, (Figure 3a) and this occurred even prior to catalysis, during the
catalyst preꢀheating (Figure S5). Lastly, 0.5 PW12@NUꢀ1000 also lost integrity of the POM
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cluster, as shown by the loss of the features at ꢀ15 and ꢀ16 ppm, and their replacement by
broad features between ꢀ10 and ꢀ14 ppm (Figure 3b). Instability of POM in harsh catalytic
55ꢀ56
conditions as well as deactivation has been reported for many other POM systems.
Although not listed in Table 2, similarlyꢀsynthesized catalysts with POM loadings from 0.3 to
.7 POM per Zr node also showed no activity at any time, and for none of these was the
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longꢀrange order maintained after the catalytic test (Figure S6). In contrast, the crystal
structure of the bare NUꢀ1000 support was retained after catalytic conditions (Figure S7), and
likewise, the pore dimensions (Figure 2b), longꢀrange crystal (Figure 3a) and POM structure
(Figure 3b) of the catalytically active 0.9 PW @NUꢀ1000. Maximal loading of POM within
12
the NUꢀ1000 framework appears to synergistically stabilize both the POM and the MOF
structure.
The absence of reactivity for low loadings may have analogies to other studies in which,
for example, catalysts of low POM loadings on silica support show less activity for alkane
isomerization. In such cases, isolated POM clusters coordinate with three surface silanols,
57ꢀ58
leaving no available protons for acid catalysis.
In contrast, completely filling the MOF
with POMs appears to physically prop open the catalyst structure in much the same way that
59ꢀ61
one would pillar a clay material,
preventing loss of longꢀrange order and catalytic
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2
reactivity. PTA is known to have a particularly strong interaction with ZrO
2
,
which may
cause the MOF to collapse around the POM if physically able. Although the POM clusters
appear to be uniformly distributed within the NUꢀ1000 (Figure S1), we cannot discount the
idea that the POM cluster occupy differentlyꢀreactive sites in the NUꢀ1000 at low and high
loadings. Unfortunately, the complete collapse of the pore structure and reactivity at low
POM loadings makes challenging a further understanding of the potential structural
heterogeneity of the asꢀsynthesized catalysts.
Although having good initial reactivity, 0.9 PW12@NUꢀ1000 deactivates gradually over
time at 523 K, as shown in Figure 5a. Some activity is regained upon moving to a higher
temperature, but the catalyst resumes deactivating immediately. This is similar behavior to
63,64,65
many other microporous solid acid catalysts for CꢀC isomerization,
and it is generally
ascribed to carbon deposition or strong adsorption of reaction intermediates on active sites.
Further study and optimization of reaction conditions may be able to minimize its impact. As
already noted, Figure 3a shows that this deactivation is not due to wholesale loss of the MOF
longꢀrange order, and Figure 3b confirms that the POMs remain intact, although there may be
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some reorganization of POMs within the pores of the support. Also, Figure 2a and 2b indicate
that mesoꢀ and micropores of NUꢀ1000 are still present, and that the mesopores in particular
have not changed in dimensions. Mass balance for this reaction is good (higher than 95%),
excluding excessive coking, and there is no change to the physical appearance of the catalysts
after the reactions.
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While the organic ligands of the MOF prevent regenerating the catalyst by calcination, as
well as precluding analysis of coke by TGA, no species were detected upon purge under inert
flow at 573 K. This observation rules out the possibility of deactivation due to simple
condensation of species within the pores.
Figure 5. Representative timeꢀonꢀstream: (a) conversion of oꢀxylene and (b) selectivities of
products (♦: toluene, □: pꢀxylene, ○: mꢀxylene, +: 1,2,4ꢀtrimethylbenzene) over 0.9
PW12@NUꢀ1000 at 523 K (before 240 min) and 573 K (after 240 min).
1
3
Solidꢀstate C CP MAS NMR spectrum (Figure 6) of the used catalyst after inert purge
73 K shows a resonance at 17.8 ppm consistent with a mobile ArꢀCH remaining within the
, identified, and quantified by
5
3
2 2
catalyst. This species was successfully extracted with CD Cl
1
solution H NMR as approximately 2 oꢀxylene molecules per POM cluster (Figure S8). Thus,
the decrease in rates may be due to a slow approach to a steady state when the POMs become
saturated with strongly bound reactants. Rate laws will be fully developed in future
investigations over these materials.
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Figure 6. Solidꢀstate C CP MAS NMR spectra of 0.9 PW12@NUꢀ1000 as synthesized
black) and after catalysis (blue). + indicates spinning sidebands, * indicates new feature
assigned as adsorbed (C )(CH
(
6
H
4
3 2
) .
In addition to differences in rates, the catalysts promote different reaction pathways.
Scheme 1 shows two representative oꢀxylene reaction pathways that different materials can
2
promote. In the intramolecular isomerization pathway, oꢀxylene is protonated at a Brønsted
acid site. Then, a methyl group can shift from ortho to meta and further to para. At the low
conversions used in these experiments, the selectivity to pꢀxylene is expected to be low, since
two methyl shifts would be required before desorption from the active site. The experimental
absence of pꢀxylene in these experiments also rules out direct ortho to para transitions that
6
6
67
have been proposed for an intermolecular pathway or from an extremely active catalyst. In
the intermolecular mechanism, two reactant molecules disproportionate to yield toluene and
trimethylbenzene. The experiments strongly favor 1,2,4ꢀtrimethylbenzene over formation of
3, 68
1,2,3ꢀtrimethylbenzene, as consistent with previous studies.
Given the low conversion, the
essentially zero yield of 1,3,5ꢀtrimethylbenzene is also expected. The disproportionation
reaction is favored in materials with two adjacent active sites to accommodate migration of
the proton from one ring to another. This mechanism has been reported to occur primarily in
highly porous materials such as zeolites with relatively large channels (FAU, MOR, ZSMꢀ12),
1
ꢀ2
where the xylene species are present in high local concentration. Min et al. experimentally
verified the formation of the diphenylmethaneꢀbased intermediate in zeolites that have large
6
9
cavities or channels, and showed up to 99% disproportionation selectivity in such materials.
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Scheme 1. oꢀxylene reaction pathways: intramolecular isomerization and intermolecular
disproportionation
9
The WO ꢀZrO reference exhibits the typical behavior for this catalyst, with 100 %
x
2
selectivity toward the isomerization pathway in differential conversion regime. In contrast,
the product selectivity over 0.9 PW12@NU1000 shows primarily oꢀxylene disproportionation,
where 83% of the product consists of a near equimolar mixture of toluene and
trimethylbenzene, with the remaining minority product being mꢀxylene. The high selectivity
to the disproportionation products and the apparent strong chemisorption of oꢀxylene leads to
the proposed catalyst structure of Figure 7. In this hypothesized structure for the working
52, 70ꢀ71
state of the catalyst, the POM is electrostatically bound to the node
via one of the three
+
H of the POM. Recently, our team demonstrated via a crystallographic method that the HPA
72
6
and the Zr node are close enough to be able to form a hydrogen bond, which we denote in
Figure 7. We also note that at reaction temperatures, we would expect some mobility and
disorder of the POM location within the support framework, leading to deviations from the
+
crystallographicallyꢀdetermined structure. The remaining two H lead to two chemisorbed oꢀ
xylene per POM, as seen in the extracted solution after catalysis (Figure S8). The
chemisorption of oꢀxylene may be unusually strong because of the additional stabilizing
influence of the MOF aromatic ligands, especially given the PXRD pattern evidence that the
POM is likely found within the small triangular channels. We hypothesize that the location of
the POM within the MOF structure provides unique environments for the two reaction
pathways. The space between the POM and an adjacent linker (in the ab plane) only provides
room for a single oꢀxylene (marked in orange in Figure 7), which could promote the
isomerization pathway. In contrast, the triangular channels (above and below POMs along the
6
cꢀaxis) and the lateral cavities (between the two adjacent Zr nodes along the cꢀaxis) appear
to be appropriately sized for the intermediate of the disproportionation pathway
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(trimethylated diphenylmethane, marked with purple in Figure 7). These triangular channels
are 1.2 nm, comparable to those of a largeꢀpore acid zeolite, in which disproportionation is
69
known to occur.
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Figure 7. Proposed structure of PW12@NUꢀ1000. Color coded for potential reaction sites for
isomerization (orange) and disproportionation (purple) under reaction conditions (right) and
proposed hydrogen bond interaction under ambient condition (left bottom dotted circle). Dark
5
green=Zr, Blue prisms=WO , red=O, gray=C, white=H
■ CONCLUSIONS
The Keggin POM of phosphotungstic acid was encapsulated in NUꢀ1000 (PW12@NUꢀ1000),
and this catalyst was shown to be active for the benchmark strong acidꢀcatalyzed reaction of
oꢀxylene isomerization/disproportionation at 523K. While MOFs display stable activities for
many reactions, researchers have predominantly used MOFs as precursor, i.e. with deliberate
3
7, 73ꢀ75
decomposition of the framework,
for use under aggressive conditions. This work
demonstrates that MOFꢀsupported catalysts can be structurally stable during CꢀC skeletal
rearrangement reactions that have been widely used in industrial and academic investigations
of porous solid acids, but never previously in MOFs. Indeed, maximal loading of ~1 Keggin
unit per unit cell of NUꢀ1000 appears to stabilize structures of both the PTA and NUꢀ1000;
lower loadings of PTA immediately lost all structural integrity under reaction conditions.
Furthermore, WO
x
installed by other methods, such as impregnation of tungsten amido
sites that are unreactive for this reaction. Finally, the
complexes, gives isolated WO
x
structurallyꢀstable PW12@NUꢀ1000 catalyst shows strongly atypical selectivity toward
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disproportionation, which likely results from the particular pore structure of MOF support in
which two Keggin units are stabilized in close proximity in the smaller (but still large relative
to an aluminosilicate zeolite) side channels of the MOF. In the future, we expect that the
tunable pore structure of MOFs via synthesis with different linkers will lead to new
opportunities for controlling reaction pathway selectivity by placing the PTA in different pore
environments or at different local densities. This control might enable a level of reaction
network tuning which is not readily accessible with conventional oxide materials.
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■ EXPERIMENTAL METHODS
Synthesis of Catalysts. The catalyst support, NUꢀ1000, was synthesized and activated
76
following the published literature. The control material WO
x
ꢀZrO
2
was synthesized via
8
incipient wetness impregnation and calcined according to a protocol in the literature.
Phosphotungstic acid (H
addition of PTA (2 eq. PTA to Zr
3
PW12
O
40) was encapsulated in NUꢀ1000 (PW12@NUꢀ1000) via
node for 0.9 PW12@NUꢀ1000 and 0.5 eq. PTA to Zr node
6
6
for 0.5 PW12@NUꢀ1000) dissolved in water following a procedure previously developed by
48
our team. For the synthesis of WꢀSIM (Solution phase grafting in MOFs), 200 mg of NUꢀ
6 6
1000 (92.6 ꢁmol of Zr node) and 235.1 ꢁL (8 tungsten equivalents to one Zr node) of
bis(tertꢀbutylimido)bis(dimethylamino)tungsten (VI) (Strem, min. 97% BTBMW) were
dissolved in 5 mL heptane (99 %, anhydrous, Sigma Aldrich) in an argonꢀfilled glove box for
24 h with occasional swirling. The mixture was decanted, and then mixed 5 mL of fresh
heptane for 4 times to remove unreacted tungsten precursor. After this washing step, the
material was transferred out from the glove box, and then a similar washing step was repeated
but with acetone. Upon exposure to moist air, both amines and imines ligands from the
77
tungsten precursor were removed to form tungsten oxide clusters on the nodes of NUꢀ1000.
Characterization of Catalysts. Tungsten (209.475, 209.860, 224.875, and 239.709 nm) and
zirconium (327.305, 339.198, 343.832, and 349.621 nm) loadings were collected using
inductively coupled plasma atomic emission spectroscopy (ICPꢀAES, iCAP 7600, Thermo
Scientific). One mg amounts of catalysts were digested in 0.75 mL sulfuric acid (ACS
reagent, 95ꢀ98 %, Sigma Aldrich) and 0.25 mL of 30 wt.% hydrogen peroxide (aqueous,
Sigma Aldrich) solution with the aid of microwave using Initiator+ (Biotage) at 150 °C for 5
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min.
Nitrogen physisorption isotherms at 77 K were collected using a Tristar II (Micromeritics),
and DFT (Density Functional Theory) pore size distributions were derived from the
desorption isotherms. Prior to the measurement, samples were dried at 120 °C under dynamic
vacuum for 18 h using a Smart VacPrep (Micromeritics).
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Powder Xꢀray diffraction (PXRD) patterns were collected on a SmartLab diffractometer
(Rigaku) with a CuꢀKα radiation source (45 kV and 160 mA for tube voltage and current,
respectively). Diffraction patterns were recorded over the 2θ range of 2–20° with a 0.05° step
width and a 5°/min scan speed.
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Solidꢀstate CP MAS C and P NMR spectra were collected on a 400 MHz VNMRS
spectrometer (Varian) equipped with a conventional triple resonance 5 mm probeꢀhead in
double resonance mode. Samples were packed in a zirconia rotor, and the spinning frequency
was set at 10 kHz. Chemical shifts (δ [ppm]) were referenced to adamantane and
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1
(NH
4
)H
2
PO
4
for C and P, respectively. Solution H NMR spectra were recorded on an
Cl (Cambridge
AVANCE III 600 MHz (Bruker). Used catalysts were washed with CD
2
2
Isotope Laboratories, D 99.8 %) to extract organic molecules from the samples. Spectra were
obtained applying a 30° pulse, an acquisition time of 2.7s, a delay time of 5 s (after finding
that the much longer delay value of 20 s did not affect the area ratios between STD and
analyte), and accumulating 16 scans. Spectra were carefully phased and baseline corrected.
Quantification was performed comparing the areas of the peaks of the reference compound
(1,4ꢀdimethoxybenzene) and the target compound (oꢀxylene). Spectra were collected at the
IMSERC (Integrated Molecular Structure Education and Research Center) facility at
Northwestern University.
3
W L ꢀedge Xꢀray absorption spectroscopy (XAS) was performed at sector 5 of the
Advanced Photon Source, Argonne National Laboratory, on the DupontꢀNorthwesternꢀDow
Collaborative Access Team (DNDꢀCAT) bending magnet D beamline using a Si(111) double
crystal monochromator. All measurements were performed in transmission mode with
Canberra ionization chambers. Energies were calibrated against a W foil reference spectrum
collected with each sample spectrum by setting the W foil first inflection point at the known
3
W L ꢀedge to 10207 eV. Catalysts pellets were pressed into specially made Al sample holders,
dried under vacuum at 120 °C, then cooled and switched to flowing He before data collection.
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ꢀ
1
Scans were performed with a 0.6 eV increment in the preꢀedge region and 0.05 Å intervals
ꢀ1
ꢀ1
between 3.0 Å < k < 14.0 Å . Counting time per data point was increased from 1 to 5 s
from beginning to end of scans to maximize signal count at high k. XAS spectra were
processed in Athena using standard methods. Spectra were normalized by fitting a cubic
polynomial to 150ꢀ700 eV above the edge and subtracting the expected absorption at the edge.
The background was removed using the AUTOBK algorithm with R=1.0 and spline range
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ꢀ1
ꢀ1
from 0.5 Å < k < 13.5 Å when computing χ(k). The Fourier transform was performed on
2
ꢀ1
ꢀ1
the k ꢀweighted χ(k) function over the range 3 Å < k < 12.8 Å using a Hanning window
ꢀ1
function with width dk = 0.5 Å .
Catalysis. Kinetic studies were carried out in a flow reactor at 523 K using 50ꢀ100 mg of
catalysts to maintain differential conversion regime (< 10%). Catalysts that were supported
on quartz wool were loaded in a quartz tube reactor. The reactor was heated to 523 K with a
2
ramp rate of 5.0 K/min under flow of H (10 mL/min, 99.999%, Airgas). oꢀxylene (≥ 99.0%,
SigmaꢀAldrich) was introduced by Ar (10 mL/min, 99.999%, Airgas) bubbler with a total
pressure of 1 bar at 298 K (0.87 kPa of reactant). Reactions were carried out at 523 K for 4 h,
and then heated at 5.0 K/min to 573 K for 4 hours under same flow condition. Subsequently,
only Ar (20 mL/min) was supplied throughout the system to remove physisorbed species for
6
h at 573 K. Gas flow rates were controlled by mass flow controllers (Altamira Instruments).
Online products identification and quantification were performed by a 7890 A GC (Agilent
Technologies) equipped with a flame ionization detector (FID) and HPꢀINNOWAX (30 m
length, 0.32 mm diameter, 0.15 mm film). The mass balance with respect to the products and
unreacted oꢀxylene for all runs exceeded 95%. Initial turnover frequencies were calculated
based on conversions extrapolated linearly to time at 0 min from the trend of timeꢀonꢀstream
oꢀxylene conversion between 80 and 240 min. The reaction and online GC analysis were
performed in Clean Cat core facility at Northwestern University.
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■ ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at
DOI: XXX. Additional characterization data and kinetic data (PDF)
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■
AUTHOR INFORMATION
Corresponding Author
* Email: oꢀfarha@northwestern.edu, jꢀnotestein@northwestern.edu
Author Contributions
The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
Authors gratefully acknowledge the financial support from the Inorganometallic Catalyst
Design Center, an EFRC funded by the DOE, Office of Basic Energy Sciences (DEꢀ
SC0012702). S.L.N. acknowledges the financial support from the National Science
Foundation Graduate Research Fellowship under Grant No. DGEꢀ1324585. Metal analysis
was performed at the Northwestern University Quantitative Bioꢀelement Imaging Center.
This work made use of the J.B. Cohen Xꢀray Diffraction Facility supported by the MRSEC
program of the National Science Foundation (DMRꢀ1720139) at the Materials Research
Center of Northwestern University and the Soft and Hybrid Nanotechnology Experimental
(SHyNE) Resource (NSF ECCSꢀ1542205, NSF DMRꢀ0521267). This work also made use of
the IMSERC at Northwestern University, which has received support from the Soft and
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Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCSꢀ1542205); the State
of Illinois and International Institute for Nanotechnology (IIN). The CleanCat Core facility
acknowledges funding from the Department of Energy (DEꢀSC0001329) used for the
purchase of the Altamira BenchCat 4000. This research used resources of the Advanced
Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated
for the DOE Office of Science by Argonne National Laboratory under Contract No. DEꢀ
AC02ꢀ06CH11357.
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